Didnt know I've got this problem until I started driving my 1983 W126 (280SEL) harder than normal. Basically I'm a very mild driver, maxxing at 3000RPM. So one day I wanted to test my car's capability at higher RPMs, but once I reached 3500RPM, the exhaust starts spoofing-spoofing, as if it'd ran out of breadth. At this point in time, the power surge flattens out and if I held on at more than 3500RPM, I'm sure the power will fall. It has happened everytime I had a go at the pedal, so it is not an isolated case.